PROTOPLASM 41 



cipitation membranes. The high permeabihty of such membranes to 

 strong electrolytes is, however, against the assumption that they can be 

 primarily responsible for the selective properties of the living cell's 

 surface (Gray) ; moreover, protein membranes do not exhibit the neces- 

 sary osmotic properties. 



As a result of such difficulties, physiologists have been forced to the 

 conclusion that the membrane is characterized by a peculiar physical 

 and chemical complexity. That both lipide and protein constituents are 

 present is strongly indicated, but what other constituents may accompany 

 them, and how they are all physically arranged, is not at all clear. 

 According to Gortner (1929), it is possible to picture the membrane pro- 

 visionally as consisting of a "protein gel, probably in the form of a fibrillar 

 structure with fats, soaps, and lipides immeshed in the protein network. 

 The transfer of lipide-soluble materials would be through the fat-soap- 

 lipide portion of the structure, whereas the passage of water and such 

 water-soluble materials as actually do pass in and out of the cells would be 

 through the hydrated filaments of the protein network." 



Various conjectures have been made regarding the possible alterations 

 in the structure of the colloidal cell membrane as it undergoes observed 

 changes in permeability. Clowes (1916, 1918), who studied the relation 

 existing between balanced antagonistic salts and phase reversal in olive 

 oil-water emulsions, believed that similar reversals of lipide and aqueous 

 phases might account for permeability changes in cell membranes, the sub- 

 stance originally forming a continuous path becoming discontinuous, and 

 vice versa. Many membranes maintain their normal state of semipermea- 

 bility only when the ratio of such antagonistic salts is held constant within 

 narrow limits. Since colloidal systems are known in which the two phases 

 differ chiefly in the relative proportion of water they contain, it has 

 also been suggested that changes in permeability may be due to redis- 

 tributions of water between such phases in the membrane (Lloyd, 1915; 

 Free, 1918). Such changes would affect the size of dispersed droplets and 

 the closeness with which they are packed and hence the action of the 

 membrane as a whole. Such a hypothesis is thought to account more 

 readily for very gradual changes in permeability than does the reversal 

 hypothesis. Both gradual and sudden changes occur, and it is highly 

 probable that they involve structural alterations of more than one type. 



In his discussion of this general subject Gray (1931) lays emphasis 

 upon the fact that "the living cell is bounded by a surface or a membrane 

 which is capable of generating energy or of transforming energy which is 

 supplied to it in an appropriate form, " and upon the further fact that no 

 inanimate system known at present has the electrical properties mani- 

 fested by the cell membrane. "In other words the plasma membrane is 

 part of a dynamic system whose machinery is of a type not yet demon- 

 strated outside the living cell." It remains true, however, that the 



